STRUCTURAL CHARACTERIZATION OF VARIOUS CHIRAL SMECTIC-C PHASES BY RESONANT X-RAY-SCATTERING

We report the results of resonant x-ray diffraction at the sulfur K-ed ge performed upon free-standing films of a thiobenzoate liquid-crystal compound. Our data provide the first direct structural evidence of di stinct periodicities in several chiral Sm-C phases, including 2-layer, 3-layer, and 4-layer superlattices in Sm-C-A, Sm-C-FI1*, and Sm-C-FI 2, respectively. In Sm-C-alpha*, periodicity incommensurate with the layer spacing was detected. The racemic compound version was also stud ied. The results are consistent with a ''clock model'' of the Sm-C va riant structures.